1. Field of the Invention
This invention relates to antenna design and, more particularly, to broadband horn antennas with integrated impedance matching networks.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
An antenna is a device which can radiate or receive electromagnetic (EM) energy. An ideal transmitting antenna receives power from a source (e.g., a power amplifier) and radiates the received power into space. That is, electromagnetic energy escapes from the antenna and, unless reflected or scattered, does not return. A practical antenna, however, generates both radiating and non-radiating EM field components. An example of a non-radiating EM field component would be the portion of the accepted power that is returned to the source, or otherwise dissipated in a resistive load.
The performance of an antenna can be characterized in a variety of ways. First, the radiation efficiency of an antenna (or “antenna efficiency”) can be defined as the ratio of the amount of power radiated by the antenna to the amount of power accepted by the antenna (from a power source). The portion of the power accepted by the antenna, but not radiated, may be dissipated in the form of heat. Other antenna performance characteristics include radiation pattern, operating frequency bandwidth, gain and directivity.
As used herein, the “radiation pattern” of an antenna may be defined as the spatial distribution of a quantity, which characterizes the electromagnetic field generated by the antenna. The radiation pattern is usually given as a representation of the angular distribution (in spherical coordinates, θ and φ, at a fixed radial distance, R, from the antenna) of one of the following quantities: power flux density, radiation intensity, directivity, gain, phase, polarization or field strength (electric or magnetic). The directivity, gain and polarization of an antenna can be computed with knowledge of the antenna's radiation pattern.
For example, the “directivity” of an antenna may be defined as that in the direction of maximum radiation. For most directional antennas, the radiation pattern includes one main lobe (pointing in the direction of maximum radiation) and several smaller side lobes (due, e.g., to reflections or cross-polarizations within the antenna). The side lobes tend to detract from the overall performance of the directional antenna by reducing the amount of EM energy radiated in the intended direction.
The “gain” of a directional antenna may be defined as the directivity multiplied by the radiation efficiency of the antenna. As such, the antenna gain will be less than the directivity for real antenna designs, which provide less than 100 percent radiation efficiency.
Electromagnetic fields are vector fields. The behavior of the vector nature of an electromagnetic field is often referred to as the “polarization” or “polarization state” of an antenna. Most antenna designs used for Electromagnetic Compatibility (EMC) testing are linearly polarized. A dual-ridged horn antenna, or tapered dual-ridged waveguide, is one example of a linearly polarized antenna in that the electromagnetic field produced by the horn on the principal axis and in the principal planes is linearly polarized. When heavily loaded, a dual-ridged horn antenna may be capable of providing a rather large operating frequency bandwidth (e.g., from about 1 GHz to about 18 GHz). The “operating frequency bandwidth” is typically defined as the range of frequencies which provide acceptable performance.
One embodiment of a dual-ridged horn antenna 100 is shown in FIGS. 1 and 2. In the illustrated embodiment, the dual-ridged horn antenna includes a pair of antenna elements 110 (otherwise referred to as “ridges” or “fins”) arranged opposite one another within a rectangular-shaped housing. Each antenna element 110 is formed having a substantially convex inner surface 112 and a substantially straight outer surface 114. The outer surfaces 114 of the antenna elements are fixedly attached to walls 120 of horn antenna 100. When coupled together, walls 120 form a rectangular-shaped cone structure having a substantially larger aperture 130 than base 140. In some cases, a rectangular-shaped box (or “cavity structure”) 150 may be coupled to the similarly shaped base 140. The cavity structure 150 is typically included to provide a shunt inductance behind the feed region of the horn antenna. The shunt inductance provides high-pass matching at the feed region and prevents energy from radiating out the back of the antenna.
As shown in FIG. 1, one or more power connectors 160 may be coupled to base 140 for supplying electrical current from a power source (not shown) to the antenna elements 110 via a coaxial transmission line (not shown). A conductive feed line 170 is included for transferring the electrical current from the coaxial transmission line to the antenna elements 110. The transition from the coaxial transmission line to the conductive feed line 170 is an important part of the horn in that it comprises part of the horn's feed region (i.e., the region or point at which power is supplied to the antenna elements). When power is supplied to the feed region, electromagnetic energy is generated and radiated out of the horn antenna. The inner surfaces 112 of antenna elements 110 are configured to guide the radiated energy as it travels from base 140, through the “throat” of the horn antenna, and out through the “mouth” or aperture 130 of the antenna.
As indicated above, some dual-ridged horn antennas are capable of operating over a rather large frequency range. For instance, some dual-ridge horn antennas used in EMC test systems are capable of providing approximately 1-18 GHz of operating frequency bandwidth. However, conventional dual-ridged horn designs are currently unable to provide a useable radiation pattern over a bandwidth significantly greater than 18:1. The bandwidth limitation is further exacerbated in quad-ridged horn designs.
A quad-ridged horn antenna is basically a dual-polarized version of a dual-ridged horn antenna and functions, in the ideal case, by exploiting the orthogonality of two modes in the quad-ridged waveguide. By maintaining the proper relation between the phases and amplitudes of the incident signals at the two ports of the quad-ridged waveguide, it is possible to produce circularly polarized far fields. More commonly such an antenna is used with a switch to provide two orthogonal linear polarizations.
In a practical situation, coupling between the two modes, especially in the feed region, is inescapable and detracts from the quad-ridged horn antenna's performance. Because of various difficulties in implementing the feed region (e.g., space constraints), quad-ridged horns have not been able to provide the same bandwidth as dual-ridged, single-polarization horns. At best, conventional quad-ridged horn antennas may provide an operating frequency range of about 1 GHz to about 10 GHz.
A need, therefore, exists for improved dual-ridged and quad-ridged horn designs that extend the usable operating frequency range beyond that which is currently available.